1. Field of the Invention
This invention relates to zoom lenses, and more particularly to a zoom lens provided, besides the photographic diaphragm for determining the F-number, with an optical stop for shading undesirable light.
2. Description of the Prior Art
In the past, many zoom lenses provided, besides the aperture diaphragm, a stop with a fixed aperture size opening. For example, U.S. Pat. No. 4,367,927 discloses a so-called 2-component zoom lens with the two components of negative and positive powers between which lies a stop to restrict part of the lower light flux of the off-axial light flux of the intermediate portion of the image, whereby coma flare is removed, and excessive vignetting is restricted. This stop, because of mainly removing the coma flare, is called the flare stop. And, ideally, the diameter of the opening of the flare stop is variable during zooming so that without mutilating the on-axial rays, that is, without giving any influence to the F-number, the object is accomplished.
Also, U.S. Pat. No. 4,190,323 discloses a zoom lens comprising negative and positive lens components followed rearwardly by a stop which moves independent from the zooming movement of the second lens component. In more detail, letting .DELTA.X2 denote the amount of movement of the second component, and .DELTA.X3 the amount of movement of the stop, the differential relation is expressed as .DELTA.X3=0.5 .DELTA.X2 so that, of the off-axis light flux of the uppermost portion of the image or the intermediate portion of the image, part of the upper light flux is restricted to improve the quality.
However, in these prior known zoom lenses, the aperture diaphragm moves as a unit with the second lens component during zooming. Therefore, in order to maintain a constant full open aperture value during zooming, because the maximum diameter of the aperture opening is determined at the telephoto end of the zooming range, and also the effective diameters of the lenses just in front and just behind the diaphragm in the second lens component are determined at the telephoto end, there is a need to partially close down the diaphragm for the full open aperture in zooming positions other than the telephoto end.
Most of the present diaphragm mechanisms have, as shown in FIGS. 1 to 3, their opening shape defined by the blades becoming a true circle only when the size of the opening is maximum, and other sizes become polygons. In the same drawings, reference numeral 1 identifies a diaphragm blade; 2 a pivot pin; 3 a diaphragm control ring; and 4 a body. Even if there is some manufacturing error or adjusting error of the diaphragm blades and the maximum aperture opening is not accurate in size, the full open aperture value in the telephoto end barely changes for the aforesaid reason, providing that the front and rear lenses with respect to the diaphragm have effective accurate diameters.
However, in zooming positions other than the telephoto end, the presence of the aforesaid error of the diaphragm blades deviates the full open aperture value by an amount equal to the difference between the ideal and actual values. When the size of the aperture opening deviates toward larger values, the lens becomes faster by that deviation, often causing spherical aberration to increase. The bad influence of this phenomenon is intensified as the zoom ratio increases, and becomes prominent in those positions which are somewhat zoomed from the telephoto end, or midway between the telephoto end and the intermediate region. FIG. 4 depicts variation with zooming of the spherical aberration, as will be seen in many zoom lenses. It is to be understood from FIG. 4 that a slight increase in the full open aperture value when in the positions between the intermediate and the telephoto regions results in rapid deterioration of spherical aberration.
Such a phenomenon appears in most of the zoom lenses which employ the present diaphragm mechanism.
Meanwhile, even the objective lenses and zoom lenses which have been corrected for aberrations have some residual spherical aberration. Particularly in zoom lenses, good correction stability of the spherical aberration throughout the entire zooming range is very difficult to achieve. For example, for the three zooming positions, namely, the wide angle, intermediate and telephoto ones, variation of spherical aberration is corrected. Then, the spherical aberration will be under-corrected between the wide angle end and the intermediate region, and over-corrected between the intermediate region and the telephoto end. Particularly in the large aperture high range zoom lens, these tendencies of spherical aberration are prominent.
Also when focusing is performed from infinity to a minimum object distance, the spherical aberration varies. It is very difficult to perfectly correct the variation of spherical aberration with focusing. For example, the first lens component counting from the front is moved to effect focusing. When the zoom lens having such a focusing provision is sufficiently corrected for spherical aberration with respect to an infinitely distant object, it is the spherical aberration with respect to the object at the minimum distance which tends, in most cases, to be over-corrected if the first lens component is a divergent system, and to be under-corrected if it is a convergent system.
Meanwhile, in the photographic optical systems for single lens reflex cameras, video cameras and cine cameras, the split prism type of distance measuring means is widely used.
FIG. 5A is a perspective view of a split prism, and FIG. 5B is a schematic sectional view of an optical system employing the split prism. FIGS. 6A, 6B and 6C are diagrams illustrating how to focus an objective lens by using the split prism as the distance measuring means. As illustrated in FIG. 6, the light flux used in measuring the object distance is a pair of partial light beams l1 and l2 of a prescribed incidence height from the optical axis vertically symmetrical with respect to the optical axis.
Therefore, if the spherical aberration of the optical system is perfectly corrected, a light flux any height from the optical axis is focused at the same point, permitting distance measuring by the split prism to be accurately performed. Also, even if there is more or less residual spherical aberration, the accuracy of distance measurement can be maintained at some level, provided that the paired light fluxes are the same height from the optical axis. This condition is, however, satisfied only when the center of the eye of the observer coincides with the optical axis. If the eye is out of alignment with the optical axis, the light fluxes used in distance measuring become partial light fluxes of different heights from the optical axis, making it difficult to accurately measure the distance. For example, with the photographic optical system having such a spherical aberration as shown in FIG. 7, there is a situation where one light flux l1 is of a range "b" and the other light flux l2 is of another range "c". Since the focal points of these light fluxes are different from each other, even if the image is in focus on the split prism, the split images appear to be offset, making it impossible to perform accurate distance measurement. When the center of the eye is not out of alignment with the optical axis, the light fluxes, l1 and l2 used are both of the same range "a". But such a situation is here. In most cases, light fluxes different in height from the optical axis are used.
It is to be understood from the foregoing that as most of the zoom lenses used as the photographic optical system have appreciable residual spherical aberration, this spherical aberration lowers the accuracy of distance measurement when the split prism is used for measuring distance.
An object of the present invention is to shut off undesirable light included in the photographic light flux of a zoom lens, and particularly that light flux which causes deterioration of spherical aberration.
Another object of the invention is to prevent the full open aperture value from change during zooming.
Still another object of the invention is to prevent lowering of the accuracy of distance measurement due to zooming or focusing.
A further object of the invention is to provide a zoom lens including at least two lens groups axially movable for zooming with an aperture diaphragm either in a space within, or on the object side of, the rearmost zoom lens group, wherein use is made of an aperture-fixed stop on the image side of the aperture diaphragm with a refracting system intervening therebetween so that on-axis light flux of the full open F-number passes through the margin of the aperture opening of the stop as zooming starts from the wide angle end, and operates from at least 0.9Z to Z of the entire zooming range where Z is the zoom ratio.
A further object is to provide a photographic optical system having a zoom lens the focal length of which is varied by moving at least two lens groups along an optical axis, an aperture diaphragm and a distance measuring means of the split prism type, wherein a stop for restricting only that on-axis light flux the direction of which almost coincides with the wedge direction of the aforesaid split prism in confronting relation to the aperture diaphragm with at least one lens intervening therebetween.